c++ function call by value not work - c++

Question

I have a problem with this code:
The problem is when I see the image original, is modified by "borrarFondo()" but this function is called from "segmentarHoja" and here entry img by value, but img modifies.
void borrarFondo(Mat& img){
img = ~img;
Mat background;
medianBlur(img, background, 45);
GaussianBlur(background, background, Size(203,203),101,101);
img = img - background;
img = ~img;
}
void segmentarHoja(Mat img, Mat& imsheet){
Mat imgbw;
borrarFondo(img); //borrarFondo is called from here where img is a copy
cvtColor(img, imgbw, CV_BGR2GRAY);
threshold(imgbw, imgbw, 0, 255, CV_THRESH_BINARY | CV_THRESH_OTSU);
Mat element = getStructuringElement(MORPH_ELLIPSE, Size(21,21));
erode(imgbw, imgbw, element);
vector<vector<Point> > contoursSheet;
findContours(imgbw, contoursSheet, CV_RETR_CCOMP, CV_CHAIN_APPROX_SIMPLE);
vector<Rect> boundSheet(contoursSheet.size());
int largest_area=0;
for( int i = 0; i< contoursSheet.size(); i++ )
{
double a= contourArea( contoursSheet[i],false);
if(a>largest_area){
largest_area=a;
boundSheet[i] = boundingRect(contoursSheet[i]);
imsheet=img(boundSheet[i]).clone();
}
}
borrarFondo(imsheet);
}
int main()
{
Mat imsheet;
image= imread("c:/imagen.jpg");
segmentarHoja(image, imsheet);
imshow("imsheet",imsheet);
imshow("imagen",image); //original image by amending borrarFondo
waitKey(0);
}
I don't want to change original image

Answer 1

opencv Mat is a counted reference (i.e. like std::shared_ptr, except different syntax) where copy construction or assignment does not copy. use the clone method to copy. read the documentation, always a good idea.

Answer 2

if you're doing something like this:
Mat a;
Mat b = a;
or like this:
void func(Mat m) {...}
or :
vector<Mat> vm;
vm.push_back(m);
all of it is a shallow copy. the Mat header will be a copy, the pointers inside, too.
so, e.g. in the 1st example, b and a share the same size and data members
this might explain, why passing a Mat by value still results in pixels manipulated from the 'shallow' copy.
to avoid that you will have to do a 'deep' copy instead:
Mat c = a.clone(); // c has its own pixels now.
and again, if you don't want your Mat to be manipulated, pass it as a const Mat & be very careful about how you use it, as illustrated below.
#include <opencv2/opencv.hpp>
void foo( cv::Mat const& image )
{
cv::Mat result = image;
cv::ellipse(
result, // img
cv::Point( 300, 300 ), // center
cv::Size( 50, 50 ), // axes (bounding box size)
0.0, // angle
0.0, // startAngle
360.0, // endAngle
cv::Scalar_<int>( 0, 0, 255 ), // color
6 // thickness
);
}
auto main() -> int
{
auto window_name = "Display";
cv::Mat lenna = cv::imread( "lenna.png" );
foo( lenna );
imshow( window_name, lenna );
cv::waitKey( 0 );
}
The Mat const& lied about mutability, and Lenna’s nose is correspondingly long, here marked by a big fat circle placed by the foo function above: